In isiXhosa, isiZulu, isiNdebele, the prefix um- attached to a name tells you whose person someone is. UmXhosa is a descendant of Xhosa. UmZulu is a descendant of Zulu. The logic is consistent across the language family, not arbitrary.
So: umNtu. By the same rule: a descendant of Ntu.
I am not a linguist, and this is a theory, not a settled fact. What I can tell you is that oral historians across the Bantu-speaking world name Ntu as a primordial ancestor — the root person at the base of the family tree. Abantu (people, the plural) follows the same pattern: Ntu's descendants, taken together.
The Western concept of "human" is biological. Homo sapiens is a species classification — a way of sorting organisms by shared anatomy. If this theory holds, Nguni languages may never have had a word for that. Not because the concept was too abstract, but because it was not the concept that mattered. You were not a random two-legged organism. You were someone's descendant. You belonged to a line.
Who is Ntu?
Oral traditions vary on the details. Some place Ntu as the ancestor from whom all Bantu-speaking peoples descend, carried south through the Great Lakes migrations over centuries. The broader Bantu expansion — which the archaeological record traces to the Cameroon-Nigeria border region, reaching southern Africa somewhere between 200 and 500 CE — is the context the academic record provides. The oral traditions and the archaeological record do not always agree, and I am not going to smooth over that gap.
What is consistent across many accounts is the image of a root ancestor whose name became the name for personhood itself. There is a saying that a person is given a name in association with their destiny. If that holds, being called umNtu is not a species label. It is a destiny — to carry the line of the first person forward.
Tuu and Ntu
In Seroa, part of the !Ui language branch — one of the ancient San languages spoken in southern Africa long before any Nguni migration — the word for a person is Tuu.
Say them side by side: Tuu. Ntu. The sounds are almost identical.
Mainstream linguistics has a ready answer: ancient borrowing. The San and the Nguni groups lived alongside each other for centuries — they traded, intermarried, exchanged language. That contact is where the click consonants in isiXhosa and isiZulu came from. A shared root word for "person" fits the same pattern.
There is another reading, one I cannot prove. That two populations arrived at the same sound for the same concept not through borrowing but through something older, something that predates the moment they found each other on the landscape. Language does not usually work as neatly as that idea requires. I keep thinking about it anyway.
Umntu ngumntu ngabantu — a person is a person through other people. If umntu was always a lineage word, that saying is not a proverb about being kind to strangers. It is a description of what a person literally is: someone constituted by their connection to others, going all the way back.